Bridge plugging techniques are widely applied domestically and abroad, and have advantages including high plugging speed, low cost, and easy operation, etc. The diameters of plugging particles used in bridge plugging techniques may be up to several millimeters or even several centimeters, or down to tens of micrometers. Therefore, bridge plugging techniques can attain an effect of mitigating lost circulation or completely plugging the lost circulation zone and improving bearing capacity, no matter whether the lost circulation is porous lost circulation, fractural lost circulation, or cavity lost circulation. However, the success ratio of conventional bridge plugging is very low, owing to a fact that formation fluids exist in the lost circulation zone of reservoir; the formation fluids refer to oil, natural gas, water, and their mixtures in the formation, such as reservoir water with dissolved inorganic and organic components and natural gas. It is well known that all processes of oil-gas reservoir, ranging from accumulation of organic substances to oil and gas generation, migration, accumulation and reservoir forming, and destroy of oil-gas reservoir, are related with formation water.
CN104563952A disclosed a non-cross-linked special gel plugging method, which comprises: running a plain drill pipe to 20-30 m above the top of a lost circulation zone, and spudding up and down; preparing non-cross-linked special gel plugging slurry that consists of water, special gel ZND-2 plugging agent, gelatinous fiber plugging agent, and rigid wedge plugging agent in a clean slurry tank, wherein the non-cross-linked special gel plugging slurry is prepared by adding 0.8-1.6 parts by weight of special gel ZND-2 plugging agent, 5-8 parts by weight of gelatinous fiber plugging agent, and 10-15 parts by weight of rigid wedge plugging agent in every 100 parts by weight of water and mixing the components intensively; injecting the non-cross-linked special gel plugging slurry into the lost circulation zone in the well immediately after the preparation, and holding the non-cross-linked special gel plugging slurry in still state for 4-8 h. However, the method has the following drawbacks: the plugging slurry is held in still state for gelatinization at the lost circulation zone in the well for so long time that it may be mixed with the formation fluids and thinned; consequently, the plugging slurry flows into the deep portion of the formation and results in plugging failure. Besides, the amount of solid particles is too low (the concentration of the solid phase is too low) in the plugging slurry, and the particle size distribution of the solid particles is narrow; as a result, the plugging slurry has a poor plugging effect for leaking formations that have a wide span of fracture width, i.e., it doesn't have high general applicability. The plugging slurry can't be injected to the target formation (i.e., the leaking formation) accurately when it is injected into the well bore; consequently, material waste may occur and the plugging effect is not obvious.
Wherein, the special gel is a water-soluble polymer and has a good shear-thinning property. However, if the special gel is used solely in the plugging in a case that the leaking pressure difference is high, the actuation pressure drop gradient produced by the gel slugs formed by the special gel is not enough to resist the damage of the leaking pressure difference because the actuation pressure of the special gel is low (e.g., usually is 2-3 MPa); consequently, the plugging effect is not ideal, and secondary lost circulation may occur. A technique of plugging with cement after special gel is applied is relatively matured in actual application; however, if severe lost circulation occurs in the reservoir, cement plugging usually is not used to avoid injuries to the reservoir; instead, bridge plugging is often used in such a case. However, the success ratio of conventional bridge plugging is very low, owing to a fact that formation fluids exist in the lost circulation zone of reservoir. It is difficult to add bridge plugging materials into the gel plugging slurry because the gel plugging slurry has high cohesive force after it swells; consequently, the application of the special gel is limited.
Therefore, it is of great significance to develop a plugging method with plugging slurry.